The van, with British license plates, was halted on the highway between the Serbian and Slovenian borders on Saturday evening during routine traffic control, according to the police.
Forty-two migrants were taken immediately to hospital and some were treated for carbon monoxide poisoning, health minister Milan Kujundzic was quoted by the Hina news agency as saying. He said two of the 42 were boys aged 10 to 12, while the others were between 15 and 20, according to Reuters.
“Some of them were unconscious, some vomited when they were found,” Kujundzic said. “They were in serious condition, dehydrated and apparently have not eaten for five days.”
Some 650,000 people passed through Croatia from September 2015 to March this year on the so-called Balkan route used by migrants, mainly from Syria, heading toward western Europe, especially Germany.
The route was largely shut down in March after a series of border closures.
Many migrants have turned to smugglers to help them cross borders illegally.
Austrian police found 71 dead migrants locked into a lorry in August 2015, and many officials fear another similar disaster.